Detailed Description

Bio-glues

Bio adhesives are polymeric materials which are naturally produced from a variety of organisms and have adhesive properties. The monomers of the polymeric bio adhesives consist of a variety of substances, but proteins and carbohydrates are the most common.
Biological adhesion is a phenomenon present in many biological systems. Examples of organisms that produce adhesives can include bacteria, spiders, marine tubeworms, sea cucumbers, barnacles and mussels. These adhesives are well known for being very strong, durable and ecologically safe compared with human-made substances. On top of that, some of them can be applied in an aqueous environment being impervious to water and turbulent forces, they give rise for many qualitative applications.

How do mussels attach themselves to a surface?

At the basis of our project are proteins which are normally produced from blue mussels (Mytilus edulis). Blue mussels produce adhesives comparable in strength to human-made glues . However these adhesives have extra advantages like the absence of carcinogens (formaldehyde) and their ability to sustain adhesive under water. Byssus threads of the blue mussel attach to a (underwater) solid surface due to catechols on adhesive proteins.

The mussel’s byssus is an exogenous attachment structure which was first described in 1711 (Brown 1952 ). Byssus’ main role is the attachment of the organism to surfaces. The byssus is a bundle of extracorporeal threads. This bundle is at the proximal end attached to the mussel’s byssal retractor muscle and at the distal end to a surface by adhesive plaques.
Mytilus edulis has the ability to bind to a very broad range of materials, ranging from glass, plastics, metals, wood and Teflon to biological materials, such as biological tissues, organisms, and other chemical compounds or molecules. As a result, the adhesive abilities of mussels have been an inspiration for the production of the synthetic version of this biological glue.

The mussel attached by its byssus threads (Powel, 2009)

How do mussels attach?

Mussels use a rather flexible method to attach to a surface; they spin a set of threads from an internal muscle to the surface, also known as stem. (Waite, 1983) (Waite,1992) The treads are 2 to 4 cm in length and attach to the surface by the use of plaques (diameter 2-3 mm). Thus, this plaque connects the substrate and the byssal thread. The specific adhesion is determined by the number of threads, which can go up to 30 to 50 threads per mussel. This will lead to a total attachment strength up to 400N. The plaque is divided into three sections. The first part is a pre-coating adhesive layer, which can attach to the surface. On top of this, the byssal threads are anchored in an open, cellular structure. (Benedict & Waite, 1986) The overall protection layer is formed by a “varnish”, which protects against enzymatic and chemical degradation. (Rzepecki & Waite, 1995)

The foot of the mussel (Haemers, 2003 )

Formation of the plaque and bysall threads

As well as the plaque as the threads are built of proteins, which are produced in the mussel’s foot. In the mussel’s foot there is a very complex organ with different types of glands of muscles. In a special part (the ventral groove), the thread and plaque are formed. (Pujol, 1967) This ventral groove runs over the whole foot and can be sealed off from the environment if required. (Waite, 1992) The foot can be bend in such a direction, that the ventral groove contacts the substrate.

In the sixties it was shown that as well as the thread as the plaque is formed by the interaction of secretions from three different kind of cells located in the foot. (Pujol, 1967) There are three specific regions in the foot where the different cell types are clustered together; namely the phenol gland, the collagen gland and the enzyme gland. (Tamarin) The phenol gland produces proteins for the protective varnish and for the plaque. The produced proteins are stored in membrane-bound granules, which are homogeneous, dense, spherical bodies. After the formation of the granules, they are transported to the secretion area.

The attachment to a surface in steps

If the mussel identified a suitable surface, this surface will be inspected by the tip of the mussel foot. After this inspection, the foot will sweep the surface to clean it. This cleaning is probably done to remove all the unbound matter. After the cleaning the foot become motionless for 2-5 minutes, in this time the formation of byssus thread takes place. This means that first the ventral groove contracts to squeeze out all the seawater. The groove is then filled again by the already produced granules, containing the proteins and all the components for the formation of the byssal threads and the plaques.

If this procedure it disturbed within 1 minute, the mussel will detach, leaving behind a plaque which is attached to unfinished thread. (Maheo, 1970) This suggests that the characteristic time for plaque formation is less than a minute. This immediately raises the following question: how is it possible that the content of the granules from the enzyme gland mix fast enough with the proteins released by the phenolic gland? This implies that the cross-linking must be very effective, within a minute, since this is the time the mussel needs for formation of the plaques. (Haemers, 2003)

One should also keep in mind the environmental and other biological influences; both environmental factors (salinity, temperature, pH, season, and substratum choice) and biological factors (age and metabolic state of the organism) also affect the efficiency and strength of byssal attachment to surfaces (Van Winkle 1970 ; Crisp et al. 1985; Carrington 2002).